1. Field of the Invention
The present invention relates generally to the field of disc drives, and more particularly to an apparatus and method for providing a reliable, ferrofluidic seal between a hub and a shaft of a spindle motor in a disc drive.
2. Description of the Related Art
Disc drives are capable of storing large amounts of digital data in a relatively small area. Disc drives store information on one or more recording media. The recording media conventionally takes the form of a circular storage disc, e.g., media, having a plurality of concentric circular recording tracks. A typical disc drive has one or more discs for storing information. This information is written to and read from the discs using read/write heads mounted on actuator arms that are moved from track to track across surfaces of the discs by an actuator mechanism.
Generally, the discs are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft fixed to a base plate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the base plate to rotate the hub relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the hub and the shaft.
Generally, as the hub rotates, lubricating fluids or greases used in the bearings tend to give off contaminates such as aerosols or vaporous components that may migrate or diffuse out of the spindle motor and into a disc chamber in which the discs are maintained. In addition, the vapor may transport particulate contaminates abraded from the bearings, or other components of the spindle motor, into the disc chamber. The contaminates may be deposited on the read/write heads and the surfaces of the discs, causing damage to the discs and/or the read/write heads as they pass over the discs. To prevent contaminant migration, the spindle motor usually includes a fluidic seal (i.e., ferrofluidic seal) to seal interfacial spaces between the hub and the shaft.
Ferrofluidic seals generally consist of a magnetic fluid, an axially polarized annular magnet, and two magnetically permeable annular pole pieces attached to opposing faces of the magnet. The magnetic fluid is conventionally composed of a suspension of magnetically permeable particles suspended in a fluid carrier. Generally, the magnet and the pole pieces are fixed to the hub and extend close to but do not touch the shaft. Magnetic flux, generated by the magnet, passes through the pole pieces and the shaft, which is also magnetically permeable, to magnetically hold the magnetic fluid in gaps between the pole pieces and the shaft, thereby forming a ferrofluidic seal.
Unfortunately, although this design has worked well for conventional lower rotational speed spindle motors, increased data capacity, along with the demand for faster disc drive information access, has caused the disc drive rotational speeds to increase, often exceeding 13,000 revolutions per minute (rpm). The centrifugal forces developed at such high speeds often exceed the ability of the ferrofluidic seal's magnetic flux to hold the magnetic fluid against the shaft due to the velocity gradient across the magnetic fluid, often resulting in the failure of the ferrofluidic seal. Further, as the rotational speed increases, the centrifugal force exerted on the magnetic fluid of the rapidly rotating ferrofluidic seal increases, thereby increasing the probability of seal failure. In addition, a sufficient centrifugal force on the magnetic fluid during dynamic operations, such as data access, can impair the ferrofluidic seal, causing splashing, and migration of the magnetic fluid, which may lead to contamination of the discs and loss of information storage and retrieval. Therefore, a need exists for a method and apparatus to provide a ferrofluidic seal between a shaft and a hub that is effective and reliable even at high rotational speeds.